1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed. Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a light-transmitting electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors in each of which such a metal oxide having semiconductor characteristics is used for a channel formation region have already been known (Patent Documents 1 and 2).